1. Field of the Invention
The present invention relates to an apparatus for driving a vibrator constituting an acoustic apparatus and, more particularly, to a driving apparatus which has an output impedance which appropriately changes in accordance with a frequency, and can cause an acoustic apparatus equivalent to a conventional one to radiate an acoustic wave having better frequency characteristics or sound quality than those of the conventional apparatus or can cause an acoustic apparatus using a conventional compact cabinet to radiate equivalent or better frequency characteristics or sound quality to or than those of the conventional apparatus.
2. Description of the Prior Art
Various speaker systems are known as a conventional acoustic apparatus.
As a driving apparatus for driving a speaker unit constituting such a speaker system, a power amplifier whose output impedance is essentially 0 is used.
FIGS. 41A and 41B are respectively a perspective view and a sectional view showing an arrangement of a bass-reflex type speaker system as one of conventional speaker systems. In the speaker system shown in FIGS. 41A and 41B, a hole is formed in the front surface of a cabinet 1, and a vibrator (speaker unit) 4 consisting of a diaphragm 2 and a dynamic electro-acoustic transducer 3 is mounted in the hole. A resonance port 8 having an opening 6 and a sound path 7 is arranged below the vibrator 4. The cabinet 1 and the port 8 constitute a Helmholtz resonator.
FIG. 42 shows a simplified electrically equivalent circuit when the bass-reflex speaker system shown in FIGS. 41A and 41B is driven at a constant voltage by a power amplifier whose output impedance is 0. In FIG. 42, reference symbol E.sub.VC denotes an output voltage of a constant voltage source as a power amplifier; R.sub.VC, a voice coil resistance of the speaker unit 4; L.sub.O and C.sub.O, an equivalent capacitance (or an equivalent mass) and an equivalent inductance (or a reciprocal number of an equivalent stiffness) of a motional impedance generated when a voice coil of the speaker unit 4 is moved; L.sub.C, an equivalent inductance (or a reciprocal number of an equivalent stiffness) of the cabinet 1; and C.sub.P, an equivalent capacitance (or an equivalent mass) of the port 8.
FIG. 43 shows electrical impedance-frequency characteristics of the circuit shown in FIG. 42. In FIG. 43, reference symbol f.sub.1 denotes a resonance frequency of a first resonance system (to be referred to as a unit resonance system hereinafter) essentially formed by the motional impedances L.sub.O and C.sub.O of the speaker unit 4 and the equivalent stiffness 1/L.sub.C of the cabinet 1; f.sub.2, a resonance frequency of a second resonance system (to be referred to as a port resonance system hereinafter) formed by the equivalent mass C.sub.P of the port 8 and the equivalent stiffness 1/L.sub.C of the cabinet 1; and f.sub.3, a resonance frequency of a third resonance system essentially formed by the motional impedances L.sub.O and C.sub.O of the speaker unit 4 and the equivalent mass C.sub.P of the port 8.
Of these resonance frequencies, the frequency f.sub.3 is not associated with a sound pressure However, the resonance frequencies f.sub.1 and f.sub.2 directly influence a sound pressure. A Q value Q.sub.1 of the unit resonance system at the resonance frequency f.sub.1 and a Q value Q.sub.2 of the port resonance system at the resonance frequency f.sub.2 largely influence frequency characteristics and sound quality of an output sound pressure
When the bass-reflex speaker system is driven at a constant voltage, if the resonance frequency f.sub.2 of the port resonance system is decreased, the Q value Q.sub.1 of the unit resonance system is increased, and the Q value Q.sub.2 of the port resonance system is decreased. In this manner, the resonance frequencies and Q values have mutual dependencies. For this reason, in order to obtain flat frequency characteristics of an output sound pressure, the unit and port resonance systems must be accurately matched with each other, such that the Q value Q.sub.1 of the unit resonance system is set to be Q.sub.1 =.sqroot.3, the resonance frequency f.sub.2 of the port resonance system is set to be f.sub.2 =f.sub.1 /.sqroot.3, and so on, thus restricting a design margin.
If the cabinet is rendered compact, the equivalent stiffness 1/L.sub.C of the cabinet is increased, and the equivalent inductance L.sub.C is decreased. As a result, the Q value Q.sub.1 is increased, and the Q value Q.sub.2 is decreased. For this reason, if a conventional constant voltage driving method is employed without any modification, a normal operation of the bass-reflex speaker system is difficult to achieve. Therefore, it is difficult to make the cabinet of the bass-reflex speaker system compact without impairing frequency characteristics of an output sound pressure and sound quality.
FIG. 44 shows a negative impedance generating circuit for which an application is filed as U.S. Patent Ser. No. 07/286,869 by the present applicant. When the negative impedance generating circuit in FIG. 44 is used as a driving apparatus for the equivalent circuit shown in FIG. 42 and an output impedance is caused to include a negative resistance -R.sub.O, the voice coil resistance R.sub.VC is reduced or invalidated. Thus, the value Q.sub.1 can be decreased and the value Q.sub.2 can be increased as compared to a case wherein the speaker system is driven at a constant voltage by the power amplifier whose output impedance is 0. Thus, the bass-reflex speaker system can be effectively rendered compact.
However, in this case, if the negative resistance -R.sub.O is constant, since the values Q.sub.1 and Q.sub.2 cannot be independently set, the speaker unit or the cabinet suffers from a certain limitation when the values Q.sub.1 and Q.sub.2 are set to be desired values.
FIG. 45 shows a second example of a conventional speaker system. This acoustic apparatus is the same as a speaker system with a port disclosed in Japanese Patent Laid-Open (Kokai) Sho No. 60-98793. An internal space of a known cabinet 21 having a rectangular section is divided into two chambers 21a and 21b by a partition wall 22. Opening ports 23a and 23b are respectively provided to the outer walls of the chambers 21a and 21b. The chamber 21a and the opening port 23a, and the chamber 21b and the opening port 23b respectively form two Helmholtz resonators. The resonance frequencies of the respective Helmholtz resonators are set to be f.sub.4 and f.sub.2 (f.sub.4 &lt;f.sub.2). An opening 22a is formed in the partition wall 22. A vibrator (dynamic speaker unit) 25 is mounted in the opening 22a. A diaphragm 26 of the vibrator 25 is mounted to close the opening 22a, the front surface of the diaphragm 26 faces the chamber 21a, and its rear surface faces the chamber 21b.
FIG. 46 shows an electrically equivalent circuit when the vibrator 25 of the apparatus shown in FIG. 45 is driven at a constant voltage. In FIG. 46, a parallel resonance circuit Z.sub.1 is formed by the equivalent motional impedance of the vibrator 25. In this circuit, reference symbol r.sub.O denotes an equivalent resistance of a vibration system; L.sub.O, an equivalent inductance (or a reciprocal number of an equivalent stiffness) of the vibration system; and C.sub.O, an equivalent capacitance (or an equivalent mass) of the vibration system. A series resonance circuit Z.sub.4 is formed by the equivalent motional impedance of the first Helmholtz resonator constituted by the chamber 21a and the opening port 23a. In this circuit, reference symbol r.sub.1a denotes an equivalent resistance of the chamber 21a as a cavity of the resonator; L.sub.1a, an equivalent inductance (or a reciprocal number of an equivalent stiffness) of this cavity; r.sub.1p, an equivalent resistance of the opening port 23a; and C.sub.1p, an equivalent capacitance (or an equivalent mass) of the opening port 23a. A series resonance circuit Z.sub.2 is formed by the equivalent motional impedance of the second Helmholtz resonator constituted by the chamber 21b and the opening port 23b. In this circuit, reference symbol r.sub.2a denotes an equivalent resistance of the chamber 21b as a cavity of the resonator; L.sub.2a, an equivalent inductance (or a reciprocal number of an equivalent stiffness) of this cavity; r.sub.2p, an equivalent resistance of the opening port 23b; and C.sub.2p, an equivalent capacitance (or an equivalent mass) of the opening port 23b. In FIG. 46, reference symbol Z.sub.VC denotes an internal impedance of the vibrator 25. When the vibrator 25 is a dynamic direct radiation speaker, the internal impedance mainly serves as the resistance R.sub.VC of the voice coil, and includes a slight inductance. Reference symbol E.sub.VC denotes a constant voltage source as a driving source whose output impedance is 0. Note that the equivalent resistances r.sub.1a, r.sub.1p, r.sub.2a, and r.sub.2p have small values which can be ignored as compared to the resistance R.sub.VC of the voice coil.
FIG. 47 shows electrical impedance characteristics of the system shown in FIG. 45. In the system shown in FIG. 45, five resonance points f.sub.1 to f.sub.5 are generated by one parallel resonance circuit Z.sub.l and two series resonance circuits Z.sub.2 and Z.sub.4. Of these resonance points f.sub.1 to f.sub.5, the resonance frequency f.sub.2 by the series resonance circuit Z.sub.2 and the resonance frequency f.sub.4 by the series resonance circuit Z.sub.4 are mainly associated with the output sound pressure.
In the speaker system shown in FIG. 45, it is ideal that the output sound pressures from the opening ports 23a and 23b become equal to each other at the frequencies f.sub.2 and f.sub.4, as indicated by solid curves in FIG. 48, and are mixed to generate a flat total sound pressure between the frequencies f.sub.2 and f.sub.4, as indicated by a dotted line in FIG. 48. However, in order to achieve this, Q values must be set to be appropriate values For example, a Q value Q.sub.4 at the frequency f.sub.4 must be set to be higher than a Q value Q.sub.2 at the frequency f.sub.2.
In the conventional constant voltage driving method, a damping resistance determining Q values at the frequencies f.sub.2 and f.sub.4 is commonly R.sub.VC. Therefore, in order to adjust these Q values to appropriate values, the volumes (L.sub.1a and L.sub.2a) of the chambers 21a and 21b and the masses (C.sub.1p and C.sub.2p) in the ports can only be adjusted.
The speaker system with the arrangement shown in FIG. 45 (to be referred to as a double bass-reflex system hereinafter) is originally adopted to efficiently reproduce a narrow band as compared to normal speaker systems, and achieves this by utilizing two resonance states.
Note that f.sub.2 =80 Hz and f.sub.4 =40 Hz, and a sub-woofer having flat characteristics in a frequency range of 40 Hz to 80 Hz is assumed.
An average energy spectrum of a music is attenuated at two sides to have 200 Hz as the center, as shown in FIG. 49. Thus, in the energy spectrum of a music signal applied to this sub-woofer, a component E(f.sub.2) of the frequency f.sub.2 is generally larger than a component E(f.sub.4) of the frequency f.sub.4. In order to achieve high efficiency, a resonance at the frequency f.sub.2 or higher must be valid. An acoustic resonance tends to have a high Q value at a high frequency rather than a low frequency if a volume remains the same, and a sound pressure is proportional to an acceleration of an air vibration. Therefore, since E(f.sub.2)&gt;E(f.sub.4), the output sound pressure at the frequency f.sub.2 becomes higher than that at the frequency f.sub.4 if the resonance Q value is left unchanged.
Therefore, it is easier to validate a resonance at the frequency f.sub.2 than at the frequency f.sub.4, and is preferable in terms of efficiency. However, the fact that the sound pressure at the frequency f.sub.4 and the output sound pressure at the frequency f.sub.2 are almost equal to each other and a band from f.sub.2 to f.sub.4 is almost flat is an original condition for the speaker system. Therefore, if the resonance at only the frequency f.sub.2 is valid, the original condition for the speaker system cannot be satisfied, and flat frequency characteristics cannot be obtained. In order to obtain flat frequency characteristics, unless a resonance at the frequency f.sub.4 is performed under a more effective condition than that at the frequency f.sub.2, the sound pressure at the frequency f.sub.4 which tends to be low is decreased.
For these reasons, in an actual double bass-reflex system, the sound pressure at the frequency f.sub.4 is increased by establishing (the volume of the cavity 21a)&gt;&gt;(the volume of the cavity 21b). The volume of the cavity 21a and the dimensions of the opening port 23a are designed to have a relatively small Q value at the frequency f.sub.2 so that the sound pressure at the frequency f.sub.2 matches with that at the frequency f.sub.4. This is to satisfy a frequency characteristic condition which is the prime importance as the performance of the speaker system by all means. Of course, such a speaker system can have improved efficiency as compared to a speaker system with no port. However, since the sound pressure by the resonance at the frequency f.sub.2 is caused to match with that at the frequency f.sub.4, efficiency at the frequency f.sub.2 is inevitably decreased. The dimensions of the speaker system are almost determined by a design not for the frequency f.sub.2 but for the frequency f.sub.4. Therefore, in view of energy, the dimensions of the system are determined on the basis of the frequency f.sub.4 at which an energy less than that at the frequency f.sub.2 is applied, and the efficiency at the frequency f.sub.2 must be suppressed to match with the sound pressure at the frequency f.sub.4.
In the acoustic apparatus shown in FIG. 45 for driving the double bass-reflex speaker system at a constant voltage, since the dimensions of the cabinet are related to the Q values at the resonance frequencies f.sub.2 and f.sub.4, a design margin is small, and it is difficult to make the cabinet compact.
FIG. 50 shows an electrically equivalent circuit when a dynamic speaker unit is mounted on an infinite baffle and is driven at a constant voltage by a power amplifier whose output impedance is 0. In FIG. 50, reference symbol E.sub.VC denotes a constant voltage source as the power amplifier and its output voltage; and R.sub.VC and L.sub.VC, a resistance and an inductance of a voice coil of the speaker unit, respectively. Reference symbols L.sub.O and C.sub.O denote an equivalent capacitance and inductance of a motional impedance generated when the voice coil of the speaker unit is moved; and R.sub.O, a mechanical damping resistance. In general, R.sub.O &gt;&gt; R.sub.VC. R.sub.VC and L.sub.VC are an electrical resistance and inductance of the voice coil itself, and are non-motional impedances.
The non-motional impedance Z.sub.VC is given by: EQU Z.sub.VC =R.sub.VC +j.omega.L.sub.VC
A motional impedance Z.sub.M is given by: ##EQU1## where .omega. is the angular frequency. If the frequency is represented by f, .omega.=2.pi.f.
FIG. 51 shows electrical impedance-frequency characteristics of the circuit shown in FIG. 50. In FIG. 50, an increase in impedance in a high-frequency range is caused by the inductance L.sub.VC of the voice coil. As described above, the inductance L.sub.VC is an electrical inductance of the voice coil itself, and is not a motional impedance. Therefore, when the voice coil is placed in a magnetic circuit formed by a magnetic member and is moved therein in response to a signal, the inductance is modulated by this signal. In particular, when a high-frequency signal is input simultaneously with a low-frequency signal having a large amplitude, the inductance L.sub.VC is largely varied by the low-frequency signal, and a current of the high-frequency signal is modulated to generate a so-called IM distortion (intermodulation distortion).
A frequency f.sub.O is a resonance frequency caused by the motional impedance Z.sub.M, and is given by: ##EQU2##
When the negative impedance generating circuit shown in FIG. 44 is used as the driving apparatus E.sub.VC in the equivalent circuit shown in FIG. 50 and the circuit is driven while the output impedance is caused to include the negative resistance -R.sub.O (to be referred to as negative-resistance driving hereinafter), the voice coil resistance R.sub.VC is equivalently reduced by the negative resistance -R.sub.O.
In the dynamic speaker unit as shown in the equivalent circuit of FIG. 50, the motional impedance Z.sub.M in a low-frequency range near the resonance frequency f.sub.O is very large, and the impedance j.omega.L.sub.VC of the inductance L.sub.VC is very small. For this reason, the impedance j.omega.L.sub.VC can be ignored with respect to the motional impedance Z.sub.M. If R.sub.VC -R.sub.O =0, the output voltage of the constant voltage source E.sub.VC is substantially directly applied to the vibration system (motional impedance Z.sub.M). Therefore, the Q value of the parallel resonance circuit of L.sub.O and C.sub.O constituting the vibration system becomes 0, and the operation of the vibration system becomes a constant-speed operation, thereby increasing a driving force and a damping force. Note that if R.sub.VC -R.sub.O &gt;0, since the resistance R.sub.VC is equivalently decreased, an intermediate state between a case wherein the speaker unit is driven at a constant voltage and a case wherein the vibration system is operated at a constant speed while R.sub.VC -R.sub.O =0 can be established. The driving force and damping force of the vibration system can be increased as compared to constant-voltage driving.
However, at a frequency in a high-frequency range separated from the resonance frequency f.sub.O, the impedance j.omega.L.sub.VC the inductance L.sub.VC increased, and the impedance 1/j.omega.C.sub.O of the equivalent capacitance C.sub.O is decreased so that the motional impedance Z.sub.M is decreased. Thus, the driving current is determined by the non-motional impedance Z.sub.VC consisting of the resistance R.sub.VC of the voice coil and the inductance L.sub.VC. For this reason, when the voice coil resistance R.sub.VC is decreased by the negative resistance driving, a driving current in a high-frequency range tends to be influenced by the voice coil inductance L.sub.VC. Therefore, an adverse influence on distortion characteristics of the speaker unit due to the inductance L.sub.VC is enhanced as compared to the normal constant-voltage driving method.
In practice, the above-mentioned infinite baffle is not used, and the speaker unit is generally mounted on a cabinet. When the speaker unit is mounted on a closed baffle (cabinet), the motional impedance Z.sub.M is equivalently connected in parallel with an equivalent inductance L.sub.C of the closed cabinet A resonance frequency f.sub.OC and a motional impedance Z.sub.MC in such a practical use state are respectively given by: ##EQU3## When such a closed baffle is used, if the above-mentioned f.sub.O is replaced with f.sub.OC and Z.sub.M is replaced with Z.sub.MC, the above description made for a case wherein the infinite baffle is used can be applied.
The bass-reflex speaker system shown in FIG. 41 in which the speaker unit is mounted on the cabinet having the resonance port, causes three resonance frequencies, i.e., the first resonance frequency f.sub.1 by a parallel resonance of the equivalent inductance L.sub.C of the cabinet and the motional impedance Z.sub.M (L.sub.O and C.sub.O), the second resonance frequency f.sub.2 by a series resonance of the equivalent capacitance C.sub.P of the resonance port and the equivalent inductance L.sub.C of the cabinet, and the third resonance frequency f.sub.3 by a parallel resonance of the motional impedance Z.sub.M and the equivalent capacitance C.sub.P of the resonance port, as described above.
Of these resonance frequencies, the resonance frequency f.sub.1 directly influences a sound pressure, and Q values at the resonance frequencies f.sub.1 and f.sub.2 largely influence frequency characteristics of the output sound pressure and sound quality. In this bass-reflex speaker system, when negative resistance driving is performed, the Q value at the frequency f.sub.1 is decreased and the Q value at the frequency f.sub.2 is increased as compared to those in the constant-voltage driving. Thus, the damping force and driving force at the frequency f.sub.1 are increased, and a matching state between the speaker unit and the cabinet can be adjusted by the negative resistance -R.sub.O, thus increasing a design margin and allowing lower bass sound reproduction However, at a frequency in a high-frequency range separated from these resonance frequencies f.sub.1 and f.sub.2, a driving current tends to be influenced by the inductance L.sub.VC. Therefore, an adverse influence on acoustic characteristics, e.g., distortion characteristics caused by the inductance L.sub.VC is promoted as compared to the normal constant-voltage driving method.